Automatic frequency control system



June 6, 1950 s. FRANKl-:L

AUTOMATIC FREQUENCY CONTROL SYSTEM Filed April 29, 1948 O ww PNJ

Patented June 6, 1950 .AUTOMATIC ,FREQUENCY .CQNIRQL SYST Sidney Frankel, Forest HiilafN. an, assignent international iStandard Electric lnrlmratnn, New Xin-k, N. X., 1an:orporationlnf.v Delaware comparison .Witha .reference oscillator such for .example asa crystal-:controlled oscillator ,orssimie lar highly .stabilized reference frequency .generator.

Another principal object ,is to provide an .automatic frequency-responsive circuit including a 1 frequency discriminatory for :stabilizing Y-a'lhigh frequency master xoscillator by `Acollparison with a reference oscillator, for example acrystal-controlled oscillator, and using a Vminimum -number of high frequency or radio #frequency circuits necessary to 'effects-the stabilization.

Another object is to providean automatic-fre quency stabilizing systemnf the type employing a mastenoscillator to b e stabilized by comparison with aflxed ,frequency oscillator such as a crystal-controlled. oscillator, the. s ignallfromthe fixed frequency oscillator being phase or frequency modulated by a loW frequency modulatingsignal which, for example, maybe gan alternating current voltage Aderived'from the ilocal alternating current power source.

A feature of the invention relates to a frequency stabilizing system employing la master oscillator to'be stabilized :by comparison with a crystalcontrolledoscillator toproducezafbeat frequency. This beat frequency is passed through a frequency discriminator, detector, and narrow band amplifier combination. lIhe'master oscillator is frequency -or :phase-,modulated by a ;,1owffrequency signal, vwhich. latter signal :is/also combined l,with the zoutput` of 4the xnarrow band -iamplier to 4.=rucoduce a frequency control lvoltage :forxtheinaster oscillator.

Another feature relates to .an fautomatic fire quency control system for a master oscillator, wherein the master oscillator frequency is compared with a frequenyzmodulated reference oscillator, the frequency modulation being effected by a low frequency alternating current signal.

The resultant :beat :frequency 'between the master .50

oscillator and the reference oscillator is passed through a special vdiscriininator and filter arrangement for deriving a` signal at the samefrequency as the said modulating signal, but whose themasteroscillatorfromithereferencelfrequency offtherefereneeioscillator. o

A still further ffeaturezrelates .to ,thenoyelmircuit organizatiomwhich lisrcapableiof iuseu'n stabiliz'ing .treni7 f high rfrequencies.

SIrhe r'abave-.mentioncd zand xother :features and objects .of ithis invention :and the im-anner .of dat tainingxthem-millsecome. moreapparentand :the inventicneitself will bezbestiunderstcod, by ret erencezo thel'followingidcriptionlof .an Jemhodi. mentmf the-invention Ltakenlin .conjunction-with theeaccompanyinmdmwinga wherein,

SFig. .liis arschematic ablock; and wiring .diagram of an automatic resiliency stabilizing Larrang-ement.accordingtozthefinvention.

,Fgs. 2.and erespectlrelyfare graphs explana ttory' of :theioperjation uncertain-portions of Eig il.

Referringito Eig. Litheablock lmepresentsfany well-known typecseillator .generator :whichis to lceiusecl:as.:annaster oscillator. iQscill-atorfl may fbe ofy any rfreeerunningetype, such for fexample las a'xlystronmscillatoriof .typenort may consist lof .a iconventional egridecontrolled elec-l tron aube oscillator. rthe frequency `of eoscil later .1 is :to iberamaintained uwithin :Mery :close limits, zand :for this ipurpse .is lprovided a standard inference itreqnency generator .2 .of any iwelleknown norm. Ehe .generator 52 may. for example, `:be Ya lcrystal-cnntrolled oscillatnn cr it :may be :an mitra. high :frequency oscillator `whose frequency Lcontrclled '.for example ,byihigiheQeavitn sin anyterentthe generator is @preferably :of alle fixed .and Ahighly stabilized ,frequency itype. viIhe generator .2 is preterahly `provided with sa ahuier .axnpliiier stage ,341i any .welleknown italie, :and cianriecteii .to :the y.plate circuitl of ihnier :stage 3 iis any .nell-known formbf :reactanceztuhe i4. Ehefreactance tube @.4 hasfapnlied .thereto fallow ufreaueny :medulatris signal from the-source z5. Llhisgmodulating esige nal, or example, imay :he a ,isixtyaeucle .signal taken rflirectls' immftheilocal alternating current power ssunnly source. :By :this .arrangement the signal fromesexierator 2 :is subiected to lalsmall deer-eennphese modulationQnfrequencygrno dirla tion. ibut without any vea,apreciable -filiales@ in .its ancrage ifrenuexicy. ifrenuencyampdulated reference sisnalas'ftheniannlied Lto lanysuitable mixer: network-.5, Lauch for example as .a .crystal diode :mixerrof :the montant ftype :such .as fis con ventionallyfuscinfnltrarhiah manueller-systems. Alsoeapplied toithezmixer Misra sample .of the signal fromzthe masteroscillator L Byitheiwellknown .actionfof theseerystalimixersthere ap phaseiis aiunction cfitherequencyfdenartureg of u pears;in-Lthemutnuttthexieofsecsigmalwhich@reime-v sents the instantaneous frequency difference between the signals from devices I and 2. This difference is then passed through a discriminator network 'I which is generally represented in the drawing by the capacitor 8 and resistor 9. This frequency discriminator is designed so as to have a response characteristic as illustrated in Fig. 3, and wherein the center-part of the linear portion of the characteristic is centered at zero frequency. In other words, if the frequency of oscillator I is identical with the average frequency of the generator 2, there will be zero voltage output from the discriminator l. However, if the oscillator I deviates from this desired frequency, there is a signal at the output of the discriminator 'I. This signal is applied to a suitable detector It to recover an audio frequency component of the same frequency as that from the modulating` source 5. The signal from the detector l will lag or lead the signal from source 5, depending upon whether the master oscillator frequency I is higher orlower than the average reference frequency from generator 2.

After detection by detectorIU. the audio frequency signal is amplified to the required level by a narrow band amplifier II, the output of which is applied to the primary winding I2 of the audio frequency transformer I3. The secondary winding I4 of this transformer has connected to it a tuned circuit comprising the inductance I5 and capacitor I6. This tuned circuit is tuned to the fundamental of the audio frequency signal from amplifier H, and it also serves to restore the Wave shape of this signal to substantially sinusoidal form. Also, thetuned circuit connected to the secondary of transformer I3 shifts the signal by an :additional 90, so that while it is at the same frequency as that of the source 5, it is either in phase with the signal from source 5, or 180 out of phase therewith. The signal from the tuned circuit I5, I6, is then electrically added to and also subtracted from the original signal from source 5. is effected by means of the pushpull audio frequency transformer I'I whose primary winding I8 is fed with a sample of the signal from source 5. The tuned circuit I5, I6, is connected in divided balanced or push-.pull relation between the mid-point of .the secondary winding I5 and the mid-point between the two condensers 2li, 2 I, andthe mid-point between the two ycorresponding resistors 22, 23. The opposite ends of the secondary I9 are connected through respective rectiers 24, 25, so` as .to subject the sum anddiiference-between the voltages from the tuned circuit I5-,I6 and the source 5 to separate rectifications. The two rectified voltages are combined in additive relation and are applied to a, suitable control voltage adapter circuit or device 25 whose output is connected to the master oscillator I. If master oscillator I is a reflex Klystron, the voltage from the rectiers 24 and 25 can be applied to the repeller electrode of the Klystron to control the Klystron generated frequency in the Well-known manner. If the oscillator l is of the grid-controlled electron tube type, having a frequency control circuit including for example an adjustable capacitor or inductance, the device 26 can be two-phase type, one of the phases being supplied from the source 5, and the other phase being supplied directly from the amplifier II, thus eliminating the elements I2 through 25.

The quantitative theory of the discriminator 1 will now be described. Let the reference signal generated in reference frequency generator 2 in the absence of modulating signal be e1=ai Sin pt (l) and let the modulating voltage applied to the reactance tube be represented by em=am sin ut (2) The reactance tube produces a phase shift proportional to the Voltage pm-bm sin ut (3) A reactance tube need not necessarily be used. A certain amount of frequency shift could be obtained by varying the oscillator screen voltage, for example.

After modulation the control signal (Equation l) becomes, by virtue of (Equation 3) ei=a1 sin (pt-I-bm sin ut) (4) Let the controlled signal of master oscillator I be represented by where is the algebraic difference between p and q, and is an arbitrary phase constant. If these two signals, (Equation 4) and (Equation 5) are mixed and the difference-frequency component abstracted, the result is where w is the instantaneous angular frequency of the applied signal; en is the output voltage; and e1 is the input voltage. Then since in (Equation 6) If now We detect for the envelope of (Equation 9) We get es: {dauw-uhm cos ut) (10) Without loss of generality assume now that a3a4 0, bm 0. This can always be arranged, if necessary. The assumption places no important restriction on this proof .A 1

Assuming first a condition where [l lubml we have to consider'two cases:

es=aaa4(S|-Iubm cos ut) On removing theD. C. term through a blocking condenser we now have rapplied. to. the narrow bandamplifler F provided ll ubml Thus the phase of ee depends on the Sign of .c which is essentially what we wish to accomplish. Itis necessary, however, to take care of the im. portant condition where ||lubm| which exists as the ycontrolled oscillator falls into step with the reference generator.

For [llubml Eq. (4) (with a3a4 0bm 0) yields a result generally as shown in Fig. 2, for 0. For the picture is the same with the ordinates reversed., A Fourier analysis for the fundamental component yields The whole characteristic for es is now given by (Equation 11) and (Equation 12) as long as the discriminator is in its linear region (Equation 7') As S- O, X 0 also, and Equation 12 then shows that es also tends to zero. Thus, if we filter all but the fundamental component we obtain a control voltage that changes sign with change in sign of the frequency error, `and goes to zero with the. frequency error. The tuned circuit in the secondary of transformer G performs this lterat .the same time as it shifts the voltage back to a sine function.

The D. C. voltage developed after rectification is easily seen to be The idealized characteristic obtained with a linear discriminator is shown dotted in Fig. 3. Actually, as ll-0, the discriminator output becomes independent of frequency and the A. C. voltage developed falls asymptotically to zero as |l as indicated in the solid curve of Fig. 3. The sign of this voltage does not reverse, however. The system, is therefore suitable as an automatic frequency selection system as well.

In case the modulating voltage (Equation 2) is derived from the local power source, and in case the frequency of the controlled oscillator is adjustable by a, motor-driven mechanism, the motor can be of the two-phase type, and one of the phases can be taken directly out of the narrow band amplifier Il without further modification, and the remaining components of the block diagram of Fig. 1 are unnecessary. The other phase is taken directly from the power supply.

In the discriminator 1 it is easily shown that when the angular frequencies of oscillator and reference generator differ by the largest alternating current voltage is developed where 6l and that when the constants are Aso chosen,v then the alternatingcurrent voltage actually developed eb=..0.3.85A-pa3 cos ui (l where Ap is the frequency-modulation angular frequency deviation.

While I have described above the principles of my invention in connection with specific appara-. tus, it is to be clearly understood that .this description is made only by way of example and .not as a limitation to the scope of my invention.

What is claimed is:

1. An automatic frequency control arrange ment, comprising an oscillator whose frequency is to be controlled, a standard reference frequencyv generator, means to phase-modulate the signal from said generator by a low frequencyalternating current without changing the average frequency of the generator signal, a freev quency mixer device, means to impress upon said device a sample of the frequency from said oscillator and a sample of the phase-modulated signal from said generator to produce a beat fre-` quency, a frequency discriminator network upon which said beat frequency is impressed, a detector for detecting from the output of said dis-g criminator a component of the same frequency as said modulating frequency but whose phase with respect to said modulating frequency is in accordance with the deviation of said oscillator from its predetermined frequency, and phase comparison means, including circuit connections between said detector, said modulator and said oscillator, for deriving and applying to said oscillator a correction vol-tage controlled by the phase difference between said component and said modulating frequency.

2. An automatic frequency control arrangement, comprising an oscillator whose frequency is to be controlled, a frequency-stabilized refer-v ence frequency generator, a source of low frequency alternating current, a phase-modulating device between said source and said reference generator for phase-modulating the signal from said generator without changing the average generator frequency, a mixer device connected to the output of said generator and to said os,- cillator to produce a beat frequency when said oscillator deviates from its predetermined frequency, a frequency discriminator network connected to said mixer, a detector connected to said discriminator for detecting a component of said beat frequency which has the same frequency as said low frequency source, a combining network for combining the signal from said detector with a sample of the said low frequency to produce an oscillator control voltage corresponding to the difference in phase between the said detected component and said low frequency.

3. A frequency control arrangement according to claim 2, in which the said detector is connected to said combining network through a filter circuit which is tuned to the fundamental frequency of said detected component.

4. An automatic frequency control arrangement according to claim 2, in which said combining network is connected to said detector through a tuned circuit which is tuned to the fundamental frequency of the detected component and produces a quadrature phase relation between said detected component and said low frequency.

5. An automatic frequency control arrangement, comprising a master oscillator, a reference frequency generator, a frequency mixer device upon whichV a sample of the signals from said master oscillator and from said frequency generator are impressed, means including a source of low frequency alternating current for phase-modulating the signal from said generator without changing the average generated frequency, a frequency discriminator connected to said mixer, a detector connected to said discriminator for detecting a component of the discriminator output which has the same frequency as said low frequency source, a pair of rectiers, means to apply to said rectiers in push-pull relation a sample of the voltage from said low frequency source, means to apply, simultaneously to said rectiers in divided balanced relation, voltage controlled by said detected component but in quadrature phase with respect to said low frequency voltage, means to derive from said rectiers an oscillator control voltage corresponding to the sum and difference of said low frequency voltage and said phase quadrature voltage, and circuit connections for applying said oscillator control voltage to said master oscillator to maintain said master oscillator at a predetermined frequency.

6. An automatic frequency control arrangement according to claim 5, in which the said detector is connected to the input of a narrow band amplifier and the output of said amplifier is connected through a filter circuit to said rectiers, which filter circuit is tuned to the fundamental frequency of said detected component.

'7. An automatic frequency control arrangement, comprising a master oscillator, a reference y frequency generator, a crystal mixer upon which samples of the signals from said master oscillator and said generator are simultaneously impressed to produce a difference frequency, a source of loW frequency alternating current power, a reactance tube connected to said source and to said generator to phase-modulate the signal ternating current, a narrow band amplier con-y nected to said detector for amplifying said cornponent, a filter network coupled to the output of said amplifier and tuned to the fundamental frequency of said detected component, an audio frequency transformer having its primary supplied with current from said low frequency alternating current source, a pair of rectifiers connected in push-pull relation across the secondary of said transformer, circuit connections forconnecting said filter in divided balanced relation between the electrical mid-point of the secondary winding of said transformer and the electrical mid-point of the common output circuit of said rectiers, means to derive from the said comeA mon output an oscillator control voltage and cir--l cuit connections for applying said control voltage to said master oscillator to maintainl said master oscillator at said predetermined frequency.

SIDNEY FRANKEL.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,283,523 White May 19, 1942 2,414,100 Hansen et al. Jan. 14, 1947 2,445,663 Doelz July 20, 1948 2,452,575 Kenny Nov. 2, 1948 

